1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a hydrogen-occluding alloy pretreatment method, a hydrogen-occluding alloy pretreated by this method, and a nickel-hydrogen secondary battery employing the pretreated hydrogen-occluding alloy as an anode.
2. Description of the Related Art
Since hydrogen burns to become water and, unlike a fossil fuel, does not produce a carbon oxide gas, hydrogen can be used as a clean energy source. In an electrochemical process, hydrogen can generate electrical energy, i.e., discharging, by oxidizing into water. If the electrical energy is applied to the water, i.e., charging, hydrogen can be restored. In this case, if oxygen is used for a cathode, hydrogen and oxygen are mixed in a vessel unless a specific battery structure is employed. To prevent this phenomenon, a nickel-hydrogen battery using nickel as a cathode and a hydrogen-occluding alloy for an anode has been used.
Particularly, a nickel-hydrogen secondary battery, using the hydrogen-occluding alloy as an anode active material, is more widely applied than a nickel-zinc battery or a nickel-cadmium battery. This is because the volume energy density of the battery can be increased, since the hydrogen-occluding alloy exhibits little change in its form and exhibits a high efficiency of active material utilization, and the specific weight thereof is larger than that of cadmium (Cd) or zinc (Zn).
The nickel-hydrogen secondary battery includes a nickel cathode, an anode formed of a hydrogen-occluding alloy, an electrolyte that is an alkali aqueous solution, and a separator which is highly hydrophilic and stable in the alkaline electrolytic solution.
The reactions at the cathode and anode of the nickel-hydrogen secondary battery during charging can be represented by:
Cathode: Ni(OH).sub.2 +OH.sup.- .fwdarw.NiOOH+H.sub.2 O+e.sup.- PA1 Anode: M+H.sub.2 O+e.sup.- .fwdarw.MH+OH.sup.- PA1 Overall battery reaction: Ni(OH).sub.2 +M.sup.- .fwdarw.NiOOH+MH
where M denotes a hydrogen-occluding alloy.
The hydrogen-occluding alloy, which is capable of reversible occlusion and release of hydrogen, occludes hydrogen generated at the anode at the time of charging and releases the hydrogen at the time of discharging, so that electrochemical consumption of hydrogen occurs on the surface thereof.
General hydrogen-occluding alloys are largely classified into AB series alloys such as TiFe, AB.sub.2 series alloys such as ZrMn.sub.2, ZrV.sub.2 and ZrNi.sub.2, AB.sub.5 series alloys such as CaNi.sub.5, LaNi.sub.5 and MmNi.sub.5 (Mm: a misch metal; a group of rare earth elements like La or Ce) and A.sub.2 B series alloys such as Mg.sub.2 Ni and Mg.sub.2 Cu, where A represents an exothermic metal and B represents an endothermic metal. If the hydrogen-occluding alloy is intended to be used for a battery, its hydrogen occluding capacity is preferably excellent, equilibrium pressure between occlusion and release of hydrogen is preferably between about 0.1 to 5 MPa, and it preferably has a strong corrosion resistance.
However, among active materials in the form of the hydrogen-occluding alloy forming the anode of a nickel-hydrogen battery, metal components such as Co, Mn or Vd dissolve easily in an alkali solution as impurities. Thus, if the hydrogen-occluding alloy forming the anode is used for a long time with a strongly alkaline electrolyte, the composition of the anode changes, thereby shortening the life of the battery, which deteriorates the battery performance.
Therefore, if the hydrogen-occluding alloy is to be used as the anode material, it is desirable to pretreat the surface of the alloy.
Conventionally, the hydrogen-occluding alloy is pretreated in an alkali aqueous solution at about 80.degree. C. for several hours, that is stirred using a glass stick or mechanical stirrer having a stirring stick with a propeller. By such a pretreatment, an oxide film is removed from the surface of the hydrogen-occluding alloy and specific metal components are stripped from the alloy surface to change the composition of the alloy surface. The anode of the nickel-hydrogen battery is manufactured using the pretreated hydrogen-occluding alloy, to thereby achieve improved battery performance.
In the case of the AB.sub.5 series hydrogen-occluding alloy pretreated by etching in an alkali solution, the electrode manufactured therewith is sufficiently activated after the first 2-3 cycles and exhibits a high capacity at a high discharge current of about 1 C. That is, the high rate discharge characteristic is excellent. However, problems due to severity of an alkali etching method cannot be avoided. The Zr- or Ti-based AB.sub.2 series hydrogen-occluding alloy is not sufficiently activated by such an alkali pretreatment. So an electrode manufactured using the same requires a charge and a discharge of about 10 to 20 cycles for activation. Further, if the discharge current is increased, the capacity sharply decreases. It is believed that the sharp decrease of the capacity the Zr- or Ti-based AB.sub.2 series alloy is due to its poor electrochemical catalytic characteristic. Therefore, to solve the problem, a separate treatment must be carried out.